We just developed a similar issue with one of our IP67 gateways.
When I click on Administration->Remote Management->Check-In To DeviceHQ, I also receive Current Status: failed to open connection with the remote server.
It’s an IP67 running AEP 1.6.4.
It had been checking into DeviceHQ on a regular basis until we installed an AT&T Sim card and enabled cellular.
When we type “nslookup http://www.devicehq.com“, we get nslookup: can’t resolve ‘http://www.devicehq.com’
Any help appreciated – thanks!